Control Circuit For Transistor With Floating Source Node

ABSTRACT

A control circuit is provided for a field-effect transistor with a floating source node. The control circuit includes: a charge storage device electrically connected between a gate of the field-effect transistor and a DC power supply; a gate control circuit electrically connected between the charge storage device and the gate of the field-effect transistor; and a charge control circuit electrically connected between the DC power supply and the charge storage device. The gate control circuit is configured to receive a gate control signal and operates to turn on the field-effect transistor during on time of the gate control signal and turn off the field-effect transistor during off time of the gate control signal. The charge control circuit is also configured to receive the gate control signal and operates to charge the charge storage device with power from the DC power supply during off time of the gate control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.63/272,727, filed on Oct. 28, 2022. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to control circuit for powering atransistor with a floating source node.

BACKGROUND

When transistors are used in applications where the source or emitternode cannot be continually tied to a constant voltage, such as shortinga component located in the middle of a circuit, a floating supply isrequired to control the transistor. Typical floating supply circuitsusing switching circuitry to control transformers, inductors orcapacitors add complexity and expense to the circuit design.

If the duty cycle “ON” state time is reasonably low, the transistorcurrent requirements are low as when using field-effect transistors, thefloating source node is guaranteed to remain positive or equal to thepower supply ground, and the source node can be grounded during the“OFF” state, then a simpler floating power supply can be implementedusing a capacitor that charges during the “OFF” state and floats duringthe “ON” state. For long period applications, a rechargeable battery maybe used in place of a capacitor.

Electrical and electronic circuits generally have a power supply orsupplies that are referenced to a common node; a neutral in the case ofAC circuits or ground in the case of DC circuits. A floating powersupply is a special case whereas the ground reference of the floatingpower supply needs to be decoupled from the common ground reference ofthe other power supplies and is allowed to follow another referencesignal.

Considerations such as power requirement, size, and cost go intodesigning a floating power supply. Most all designs include a switchingcircuit and at least one capacitor; some include a transformer orinductor. One typical type of a floating power supply design involves atransformer with an electrically isolated secondary winding, a switchingcircuit controlling the primary winding and some regulation on thesecondary winding. These circuits tend to be good for applications thatrequire more power, but add considerable size and cost. Other designsuse switching circuits with an inductor and capacitor or multiplecapacitors. These circuits tend to provide moderate power, are smallerand are less expensive; but still require a switching circuit.Therefore, it is desirable to provide a control circuit for powering atransistor with a floating source node without the use of a transformeror a switching circuit.

This section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

A control circuit is provided for a field-effect transistor with afloating source node. The control circuit includes: a charge storagedevice electrically connected between a gate of the field-effecttransistor and a DC power supply; a gate control circuit electricallyconnected between the charge storage device and the gate of thefield-effect transistor; and a charge control circuit electricallyconnected between the DC power supply and the charge storage device. Thegate control circuit is configured to receive a gate control signal andoperates to turn on the field-effect transistor during on time of thegate control signal and turn off the field-effect transistor during offtime of the gate control signal. The gate control circuit furtheroperates to electrically couple a source terminal of the field-effecttransistor and a negative terminal of the charge storage device toground during off time of the gate control signal. The charge controlcircuit is also configured to receive the gate control signal andoperates to charge the charge storage device with power from the DCpower supply during off time of the gate control signal.

In an example embodiment, the gate control circuit and the chargecontrol circuit are implemented as follows. For the gate controlcircuit, a first switch is electrically coupled between the gateterminal of the field-effect transistor and ground, where the firstswitch is actuated open and close in accordance with a gate controlsignal. For example, the first switch is further defined as a transistorand the transistor electrically couples source terminal of thefield-effect transistor to ground during off time of the gate controlsignal. For the charge control circuit, a third switch is electricallycoupled between the DC power supply and the charge storage device, wherethe third switch is actuated open and close in accordance with the gatecontrol signal such that the field-effect transistor is turned on duringon time of the gate control signal and turned off during off time of thegate control signal and the charge storage device is charged with powerfrom the DC power supply during off time of the gate control signal. Itis noted that the field-effect transistor is electrically connected tothe DC power source without the use of a transformer

In some embodiments, the control circuit further includes a low-passfilter electrically coupled between the DC power source and the gateterminal of the third switch. The control circuit may also include adiode electrically coupled between the DC power source and a positiveterminal of the charge storage device.

In one application, the field-effect transistor of the control circuitis electrically coupled to an ignition coil in a vehicle.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a block diagram of a control circuit for a transistor with afloating source node in accordance with this disclosure.

FIG. 2 is a schematic of an example embodiment of the control circuit.

FIG. 3 is a schematic of the control circuit interfaced with an ignitioncircuit of a vehicle.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

FIG. 1 depicts a control circuit 109 for powering a field-effecttransistor 104 with a floating source node. The control circuit 109 iscomprised of a gate control circuit 52, a charge control circuit 51 anda charge storage device 50. In this example, the transistor 104 is anenhancement mode N-channel MOSFET although other types of transistorsare contemplated by this disclosure. The control circuit 109 connects tothe drain terminal 105 and the source terminal 106 of the MOSFET. Thecontrol circuit 109 is also part of a larger circuit that includes a DCpower supply 101 and an application circuit 110, such as an ignitioncircuit in a vehicle. For this application, the MOSFET has a requirementthat it be grounded when the transistor is in an off state but allowedto float at or positive to ground node 102 when the transistor is in anon state. Thus, the source terminal 106 of the MOSFET is selectivelycoupled to floating ground node 107.

The gate control circuit 52 is electrically connected between the chargestorage device 51 and the gate terminal of the MOSFET and is configuredto receive a gate control signal 108, where the gate control signal is arectangular pulse wave. During operation, the gate control circuit 52turns on the field-effect transistor 104 during on time of the gatecontrol signal and turns off the field-effect transistor 104 during offtime of the gate control signal. Of note, the gate control circuit 52operates to electrically couple the source terminal 106 of thefield-effect transistor to ground during off time of the gate controlsignal.

The charge control circuit 51 is electrically connected between the DCpower supply 101 and the charge storage device 50. The charge controlcircuit 51 is also configured to receive the gate control signal 108.During operation, the charge control circuit 51 charges the chargestorage device 50 with power from the DC power supply 101 during offtime of the gate control signal.

In this example, the charge storage device 50 is a capacitor 111. Thecharge storage device 50 can be any device capable of storing electriccharge. When selecting the charge storage device 50 considerationsinclude the storage capacity required, charge rate, discharge rate,component size, durability, and cost. For applications with short chargeperiods, such as on the order of milliseconds, a capacitor is typicallywell suited. For applications with longer charge periods, such as on theorder of seconds or minutes, a rechargeable battery may be bettersuited. The control circuit 109 is well suited to control a switchingdevice or other devices which require some electric charge to turn themon but minimal current to hold them in the on state.

FIG. 2 depicts an example embodiment of the control circuit 109 whichpowers MOSFET 104. The drain terminal 105 and the source terminal 106 ofthe MOSFET 104 can be electrically coupled to an application circuit asnoted above. The gate terminal of the MOSFET is electrically coupled viadiode 116 to the DC power supply. The gate terminal of the MOSFET 104 isalso electrically coupled to a floating ground node 107.

Capacitor C1 111 serves as the charge storage device. The capacitor 111has one terminal coupled to the floating ground node 107 and the otherterminal coupled to a node, where the node is interposed between thediode 116 and the gate terminal of the MOSFET 104. During operation, thecapacitor 111 stores charge when the MOSFET 104 is in an off state andsupplies charge to transition to and hold the MOSFET 104 in the onstate.

In the example embodiment, the gate control circuit 52 is comprisedprimarily of a first switch 115 and a second switch 114; whereas, thecharge control circuit 51 is primarily comprised of a third switch 111and a fourth switch 112. More specifically, the first switch 115, thesecond switch 114, the third switch 111 and the fourth switch 112 areimplemented by respective transistors. The transistors 112, 113, 114,115 in turn operate in either cutoff mode or saturation mode inaccordance with the gate control signal 108 as further described below.

The first transistor 115 is electrically coupled between the gateterminal of the MOSFET 104 and ground. That is, the source terminal ofthe first transistor 115 is electrically coupled to the gate terminal ofthe MOSFET 104 and the drain terminal of the first transistor 115 iselectrically coupled to ground. The second transistor 114 is configuredto turn on and off the first transistor 115. To do so, the gate terminalof the second transistor 114 receives gate control signal 108 and thesource terminal of the second transistor 114 is electrically coupled tothe gate terminal of the first transistor.

The third transistor 113 is electrically coupled between the chargestorage device 111 and ground. That is, the source terminal of the thirdtransistor 113 is electrically coupled to one terminal of capacitor 111and the drain terminal of the third transistor 113 is electricallycoupled to ground. The fourth transistor 112 is configured to turn onand off the third transistor 113. To do so, the gate terminal of thefourth transistor 112 receives gate control signal 108 and the sourceterminal of the fourth transistor 112 is electrically coupled to thegate terminal of the third transistor 113.

During the time the switch control signal 108 is in an off state, thefirst and third transistors 113, 115 are in an on state while the secondand fourth transistors 112 and 114 are in an off state. First transistor115 in turn pulls the charge off the gate terminal of the MOSFET 104 andkeeps the MOSFET 104 in an off state. Third transistor 113 connects thefloating ground node 107 to the ground and thereby allows the diode 116to forward bias, charging the capacitor 111. The diode 116 iselectrically coupled between the positive terminal of the DC powersupply and the positive terminal of the charge storage device so thatduring the on time of the field-effect transistor, the floating groundnode exceeds the ground voltage of the DC power supply and the chargestorage device is allowed to float with the floating ground node.

During the time the switch control signal 108 is in an on state, thefirst and third transistors 113, 115 are in an off state while thesecond and fourth transistors 112 and 114 are in an on state. In thiscase, the third transistor 113 decouples the floating ground node 107from ground and the first transistor 115 allows the capacitor 111 tosupply charge to the gate terminal of the MOSFET 104, thereby turningthe MOSFET 104 to the “ON” state. States of the transistors are shown inthe table below.

Switch Control Signal Transistors State 112 113 114 115 104 OFF OFF ON*OFF ON OFF ON ON OFF ON OFF ONIt is important that the MOSFET 104 turns completely off beforeconnecting the floating ground node 107 to the ground. In this exampleembodiment, this is accomplished by employing a first-order low passfilter 117 which slows down the activation (or turning on) of the thirdtransistor. Other techniques for achieving this timing are alsocontemplated by this disclosure.

With continued reference to FIG. 2 , the capacitor 111 needs to be sizedso that it stores enough charge so the voltage Vf 118 does not dropbelow an acceptable level before the end of the MOSFET 104 “ON” cycle.Resistor R1 is sized to provide an acceptable time-constant tau so thatthe capacitor 111 can be adequately charged during the MOSFET 104 “OFF”time. Resistor R2 needs to be sized to meet two requirements: first, itneeds to be large enough so that when transistor 115 is in saturationmode the current through resistor R2 is sufficiently limited; andsecond, it needs to be small enough so that when transistor 115 is incutoff mode and the MOSFET 104 is turning on the resistance does notoverly limit the current to the gate of MOSFET 104, excessively slowingdown the turn on time of MOSFET 104. Resistor R3 needs to be sized smallenough so that when transistor 115 turns from cutoff to saturation mode,the charge can be removed from the MOSFET 104 gate quickly, turningMOSFET 104 to the “OFF” state. The turn off time of MOSFET 104 willdetermine the time constant tau of the first order low-pass filter 117.When determining the time constant tau of the first order low-passfilter 117, the initial voltage across the capacitor C2 needs to beconsidered; it will be the voltage divider created by resistors R5 andR6 when transistor 112 is in saturation mode. Resistor R6 needs to besized small enough to guarantee that the charge is pulled off C2 beforethe next MOSFET 104 “ON” state time. Values for the capacitors andresistors in the example embodiment are listed in Table 2 below.

Component Value C1 100 μF C2 47 nF R1 100 Ω R2 282 Ω R3 12 Ω R4 20 kΩ R55 kΩ R6 50 Ω R7 500 Ω R8 20 kΩ R9 1 kΩ R10 20 kΩ R11 1 kΩ R12 20 kΩ R131 kΩ R14 20 kΩWhile an exemplary embodiment has been described above with specificcomponents having specific values and arranged in a specificconfiguration, it will be appreciated that this control circuit may beconstructed with many different configurations, components, and/orvalues as necessary or desired for a particular application. The aboveconfigurations, components and values are presented only to describe oneparticular embodiment that has proven effective and should be viewed asillustrating, rather than limiting, the inventive concept.

In this example an N-channel FET is used so the negative node of thecapacitor 111 is attached to the FET source node and the positive nodeof the capacitor 111 remains constant referenced to the negative node ofthe capacitor 111. The same principles can be applied to a P-channel FETby inverting the circuit, attaching the positive node of the capacitor111 to the FET floating source node and the negative node of thecapacitor 111 would then remain constant referenced to the positive nodeof the capacitor 111. P-channel FET designs would require other detailsto be inverted as well.

FIG. 3 illustrates an example application for the control circuitdescribed above. In this example, the control circuit 109 is interfacedwith an ignition circuit 110. Rather than having one MOSFET, the controlcircuit 109 controls the operation of two MOSFETs 104 and 119. Theignition circuit 100 includes a diagnostic circuit 121 for sampling andconditioning an ionization current signal in a combustion chamber andpower supply circuit 122 for in-cylinder ionization detection usingignition coil flyback energy. The diagnostic circuit 121 is furtherdescribed in U.S. Pat. No. 7,197,913 and the power supply circuit 122 isfurther described in U.S. Pat. No. 7,005,855; both of which areincorporated by reference in their entirety herein.

This application uses the ignition coil 123 and spark plug 124 for bothigniting the air/fuel mixture in the engine cylinder and as a path tomeasure the degree of ionization occurring during the combustion phaseof the engine cycle. The dwell control signal 125 controls the switchingtransistor 126. When the dwell control signal 125 goes high, theswitching transistor 126 turns to the “ON” state and diode 127 isforward biased causing ignition coil 123 to dwell. When the dwellcontrol signal 125 goes low, transitioning the switching transistor 126to the “OFF” state, the ignition coil 123 primary (−) node 128 flybackVoltage increases to the switching transistor 126 clamp voltage, Vclamp,typically on the order of 500 Volts, until the ignition coil 123 energyis expended. This transition causes the spark plug 124 to ignite theair/fuel mixture in the engine cylinder. The power supply circuitry 122uses the ignition coil 123 flyback energy to create the ionization biassupply voltage 129, Vion_bias. During the next phase of the engine cycle(i.e., the combustion cycle), the diode 127 is reverse biased and theionization supply voltage 129 is applied across the spark plug gap 124with the resulting ionization current being measured by the ionizationdiagnostics circuitry 121.

When the inductance of the ignition coil 123 is too high, a parasiticfilter is formed and higher frequency components of the ionizationcurrent signal are attenuated. For applications with high inductanceignition coils shorting the ignition coil 123, primary inductance can beshorted using the method described in U.S. Pat. No. 10,221,827, therebyreducing the filtering effect. The use of two MOSFETs 104 and 119 arerequired so that the body diodes prevent each other from conducting asthe polarity of the ignition coil 123 primary voltage flips betweenpositive and negative as the system moves through thedwell/ignition/combustion process. Table 3 below provides examplevoltages for the MOSFETs 104 and 119 and the drain nodes 105 and 120 asthe system progress through the dwell/ignition/combustion process.

Dwell Switch Process Control Control VPri(+) VPri(−) Phase Signal 125Signal 108 105 120 Vpri Idle OFF OFF Vion_bias 0 Dwell ON OFF 14 V GND +14 V Ignition OFF OFF 14 V 500 V −486 V Combustion OFF ON V(ionization+(<Vion_bias) current) Idle OFF OFF Vion_bias 0Further details regarding the need for a floating power supply tocontrol the MOSFETs 104 and 119 is set forth in U.S. Pat. No. 10,221,827which is incorporated by reference in its entirety herein.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A control circuit for a field-effect transistorwith a floating source node, comprising: a charge storage deviceelectrically connected between a gate of the field-effect transistor anda DC power supply; a gate control circuit electrically connected betweenthe charge storage device and the gate of the field-effect transistorand configured to receive a gate control signal, where the gate controlsignal is a rectangular pulse wave; and a charge control circuitelectrically connected between the DC power supply and the chargestorage device and configured to receive the gate control signal;wherein the gate control circuit operates to turn on the field-effecttransistor during on time of the gate control signal and turn off thefield-effect transistor during off time of the gate control signal;wherein charge control circuit operates to charge the charge storagedevice with power from the DC power supply during off time of the gatecontrol signal.
 2. The control circuit of claim 1 wherein the gatecontrol circuit operates to electrically couple a source terminal of thefield-effect transistor and a negative terminal of the charge storagedevice to ground during off time of the gate control signal.
 3. Thecontrol circuit of claim 2 wherein the gate control circuit includes afirst switch electrically coupled between gate terminal of thefield-effect transistor and ground.
 4. The control circuit of claim 3wherein the charge control circuit includes a third switch electricallycoupled between the DC power supply and the charge storage device. 5.The control circuit of claim 4 wherein the charge control circuitfurther operates to delay activation of the second switch duringtransition from on time of the gate control signal to the off time ofthe gate control signal.
 6. The control circuit of claim 1 wherein thefield-effect transistor is electrically connected to the DC power sourcewithout the use of a transformer.
 7. The control circuit of claim 1wherein the charge storage device is further defined as a capacitor. 8.The control circuit of claim 1 wherein the field-effect transistor iselectrically coupled to an ignition coil in a vehicle.
 9. A controlcircuit, comprising: a field-effect transistor having a source terminalelectrically coupled to a floating ground node; a charge storage deviceelectrically connected between a gate terminal of the field-effecttransistor and a DC power supply; a first switch electrically coupledbetween the gate terminal of the field-effect transistor and ground,wherein the first switch is actuated open and close in accordance with agate control signal, where the gate control signal is a rectangularpulse wave; and a third switch electrically coupled between the DC powersupply and the charge storage device, wherein the third switch isactuated open and close in accordance with the gate control signal suchthat the field-effect transistor is turned on during on time of the gatecontrol signal and turned off during off time of the gate control signaland the charge storage device is charged with power from the DC powersupply during off time of the gate control signal.
 10. The controlcircuit of claim 9 wherein the first switch is further defined as atransistor and the transistor electrically couples source terminal ofthe field-effect transistor to ground during off time of the gatecontrol signal.
 11. The control circuit of claim 10 wherein the thirdswitch is further defined as a transistor.
 12. The control circuit ofclaim 11 further includes a low-pass filter electrically coupled betweenthe DC power source and the gate terminal of the third switch.
 13. Thecontrol circuit of claim 9 further comprises a diode electricallycoupled between the DC power source and a positive terminal of thecharge storage device.
 14. The control circuit of claim 9 wherein thefield-effect transistor is electrically connected to the DC power sourcewithout the use of a transformer.
 15. The control circuit of claim 9wherein the charge storage device is further defined as a capacitor. 16.The control circuit of claim 9 wherein the field-effect transistor iselectrically coupled to an ignition coil in a vehicle.